The invention relates to a gas turbine and a method for protecting a gas turbine in case of a shaft break.
A gas turbine comprises a compressor, a combustion chamber, and a turbine. Depending on the type of gas turbine, multiple compressors and turbines can be provided, for example a low-pressure compressor and a high-pressure compressor, as well as a low-pressure turbine and a high-pressure turbine. The turbine is driven by combustion gases of the combustion chamber, in turn driving the compressor via a shaft. For example, a low-pressure turbine drives a low-pressure compressor via the low-pressure shaft (also referred to as an N1 shaft), and a high-pressure turbine drives a high-pressure compressor via a high-pressure shaft (also referred to as an N2 shaft).
In the case that a shaft break occurs in a gas turbine, the turbine is instantly separated from the compressor and is no longer braked by the same. At the same time, the compressor keeps conveying a mass flow for a certain period of time, by which the turbine is accelerated. Therefore, in the event of a shaft break, there is the danger of the now free-running turbine being accelerated beyond a maximally admissible number of revolutions, and of a disc brake occurring. This danger is particularly given in high-pressure turbines, which are typically built without a constructional protection against overspeed. Rather, the maximum overspeed is calculated thermodynamically, and the turbine discs are designed subsequently, wherein it is typically assumed that the maximum overspeed lies at 125% of the maximum regular number of revolutions.
Document GB 2 111 639 A describes a shaft arrangement for a gas turbine that comprises an additional shaft, which, in the case of a shaft break, moves a shaft piece of the broken shaft connected to the turbine downstream in the axial direction via a gearing in order to press the turbine into stationary structures and thus break the turbine discs into small pieces before they all fall off.
Document GB 2 377 731 A describes a shaft arrangement for a gas turbine that has an additional shaft connected to the fan. In case that the low-pressure shaft driving the fan breaks in a section adjoining the fan, the additional shaft couples with that section of the low-pressure shaft that is connected to the low-pressure turbine. The goal here is to make this section of the low-pressure shaft break as well, so that the low-pressure shaft can be axially displaced.
There is a need to provide a gas turbine and a method for protecting a gas turbine in case of a shaft break which offer that kind of protection in an effective manner.